THE DETECTION OF NITRITE AND SULFAMATE IONS IN QUALITATIVE ANALYSIS
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ROBERT C. BRASTED University of Minnesota, Minneapolis, Minnesota
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most commonly described tests for the nitrite ion are: (a) the formation of the brown ring on addition of ferrous ion and concentrated sulfuric acid to a nitrite solution; (b) the formation of a starch-iodine blue color when iodide ion is added to acidified nitrite solution; and (c) the evolution of nitrogen when urea is added to an acidified nitrite solution. The test (a) is neither specific, sensitive, nor very well adapted to student semimicro use. Test (b) is very nonspecific since any weak oxidizing agent will give the color reaction described. Test (c) resembles in principle the sulfamic acid-nitrite reaction as the following equations show.
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NH2S08H NOnHSO4Nx Hz0 (1) 2H+ NH,CONH2 2NOg-- 3H?O 2Ns GOn (2)
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The reaction represented by (2) is less sensitive to the presence of nitrite and preacidification of the solution is necessary, meaning that an unwanted anion must be added to the solution. A secondary test is suggested by the formation of the acid sulfate ion in equation (1). On adding barium chloride reagent to the reaction mixture white barium sulfate is formed if nitrite is present. The insolubility of barium sulfate in hydrochloric acid differentiates this precipitate from barium carbonate which would form if the carbonate anion were present.
acid-nitrite reaction. A number of oxidizing agents were tested for their reaction with sulfamic acid. No reaction was noted a t ordinary temperatures for the following: CrOn--, Mn04-, Br03-, I03; 1 0 4 - , and BiOn-. Anions which do react with sulfamic acid, giving an effervescence, or which might conceivably be considered as interfering with the nitrite ion, are: OCI-, OH-, S--, SO3--, Sz03--. COa--. The last mentioned interferes the most seriously. The Hypochlorite Ion, 0Cl-. A relatively slow reaction occurs between hypochlorous acid at high concentrations and sulfamic acid. The rate of reaction is much more rapid a t elevated temperatures. In practice the reaction between hypochlorous acid and the nitrite ion to form nitrate ion would take place more rapidly than the reaction between sulfamic acid and hypochlorous acid. The relative rates of the three reactions mentioned may be represented as: (NOzHSOnNH2)> (NO2- HC10) > (NH2S03H HCIO). The rapid and vigorous reaction between nitrites and sulfarnic acid cannot be mistaken in the cold for the slow hypochlorous acid sulfamic acid reaction. The Hydroxide Ion, OH-. A highly basic solution will prevent the reaction between nitrite ions and sulfamic acid for the obvious reason that the sulfamic acid will preferentially be converted to a salt. The proton must be present to initiate reaction (1). A solution basic to bromcresol green or phenolphthalein containing sodium sulfamate and sodium nitrite gives a slow but definite reaction when acidified to about 1 M with acetic acid. The SulfideIon, S--. In view of the fact that hydrogen sulfide may be evolved on the acidification of a concentrated sulfide solution, this ion must be considered as an interference. Its detection and removal by precipitation are no problem. The nitrite ion in an acid solution readily oxidizes sulfide ion to sulfur; howevert not before the nitrite ion would react with sulfamic acid if the latter were added as a test reagent. The Sulfite, SO3--, and Thiosulfate, S203--,Ions. Both of these ions will decompose in an acid solution. The effervescence is slight and would not be mistaken for that due to the nitrite ion. More serious is the reaction between the sulfite ion (or sulfur dioxide) and the nitrite ion in an acid solution (see equation (3)).
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EXPERIMENTAL Sensitivity of the Sulfamic Acid Test for Nitrites. A positive but very weak test (effervescence) for the nitrite ion is obtained when three drops of a 1M solution of sulfamic acid (or several small crystals) are added to 1 ml. of test nitrite solution containing 0.01 mg. of NaN02 (10 pg.). The test both with regard to effervescence and formation of barium sulfate is unmistakable at a concentration of 0.07 mg./ml. (70 pg.). At comparable concentrations of sodium carbonate and of sodium sulfite (about 0.001 M) no effervescence is noted. The interference of carbonate and sulfite ions is further discussed below. Tests for Interfering Ions. A number of anions were studied in order to determine which would interfere with the nitrite test using sulfamic acid. The following have been noted as not constituting any interference when present in reasonable concentrations: CzHsOz-, F-, C1-, Br-, I-, AsOa---, PO4---. SOa--, CnHlOa--, Cn04--. SCN-. CN-. &Fa--, . Fe(CN)6---, . . F e (. C N k 4 , Ljaq.), ' and ~ r i (aq:). , 2 HNOz SO* SO1-- + 2Ht 2N0 (3) Anions which in an acid solution are better oxidizing agents than nitrous acid are not listed above since a re- Whether or not the sulfite ion will completely mask the action would occur in conjunction with the sulfamic nitrite test will depend on the rapidity with which the
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NOVEMBER, 1951
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sulfamic acid test is made and the relative amounts of sulfite and nitrite ions. Unless the nitrite ion is present in nothing more than a trace, it is doubtful that either the sulfite or thiosulfate ions would constitute a serious interference. The Carbonate Ion, COs--. The carbonate ion interferes seriously with the nitrite test. If a positive l i e water test is noted on the eases evolved when sulfamic acid is added to an unknown solution, a carbonate ion concentration of a t least 10 mg./ml. is indicated. Since the nitrite ion is detectable down to a concentration of 0.07 mg./ml., it is possible t o dilute a few drops of unknown solution to a noint where effervescence is still noticeable without giving a positive limewater test. If sulfate-free sulfamic acid crystals (or solution) are used in the test, a few drops of 0.1 M BaC4 are added to the unknown solution t o act as a supplementary test for nitrite. The appearance of white barium sulfate, insoluble in HCI, in conjunction with a positive limewater test on the evolved gas, indicates the presence of both the nitrite and carbonate ions. One operation is needed then, to test for both ions. Since an unknown solution might well contain the sulfate ion, it would be desirable to make the nitrite test on the filtrate after the removal of anions forming insoluble salts with a heavy metal and the barium ion. Test for the Sulfamate Ion by the Nitrite Ion. In most anion analysis schemes used at the present time, the sulfamate ion would be included in a soluble group since all normal salts of sulfamic acid are soluble. The only ions which would interfere with the sulfamate ion (or sulfamic acid) test using nitrite ion as a test reagent would be those ions which react very rapidly with nitrites to give-an effervescence. The proton decomposes nitrites rapidly. Since the solution must be at a pH of 2-3 for the nitrite-sulfamic acid reaction to take place, the original unknown is acidified with 4 M acetic acid. Any effervescence occurring on this acidification is allowed to go to completion. Carbonates and sulfites are destroyed under these conditions. Two drops of 0.1 M NaN02 reagent solution added to the acetic acid solution give an effervescence if the sulfamate ion is present. The rate at which nitrites are decomposed by an acetic acid solution is too slow to constitute an interference by nitrite .reagent. The minimum concentration of sulfamic acid detectable is about 1.6 mg./ml. Detection of the Nitrite Ion Using Suljamic Acid. Three drops of the original unknown solution are diluted to about 3 ml. (approximately 50 to 1). Three drops of this diluted solution are treated with several small
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crystals or a drop of 1 M sulfamic acid solution. An evolution of an odorless gas strongly indicates the presence of the nitrite ion. Add two drops of 0.1 M barium chloride reagent solution to the solution after effervescence has stopped. The appearance of a white precipitate of barium sulfate, insoluble in dilute hydrochloric acid, further indicates the presence of the nitrite ion. The reactions are reuresented as:
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NO1NH,SOIH N, HS0,H20 HSOIBit++ BaSOl H +
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The supplementary barium sulfate test on the original unknown solution is obviated if the orieinal unknown contains the sulfate ion. The dilution of the original solution (50 to 1) permits the detection of nitrite at a concentration of 10 mg./ml. in the presence of 100 mg./ ml. of carbonate. If the original solution is saturated or nearly saturated with carbonate (e. g., sodium carbonate) effervescence will he noted which will mask the nitrite test. The carbonate ion is readily detected by adding sulfamic acid reagent solution or crystals to the undiluted unknown and testing the evolved gas for carbon dioxide. If a positive test is obtained for carbonate, barium ion is added to precipitate barium carbonate and the nitrite is tested for on the filtrate as described above without the dilution step. Detection of the Suljamute Ion by the Nitrite Ion. Three drops of unknown solution are treated with harium ion and a heavy metal ion (e. g., cadmium) to remove possible interfering ions such as carbonate, sulfide, and sulfite. A drop of phenolphthalein or bromcresol green indicator is added to the filtrate, now free of interfering ions, followed by the addition of two drops of 0.01 M sodium nitrite test solution. Dilute (4 M) acetic acid is added in a three-drop excess beyond the assumption of the acid color of the indicator. A rapid effervescence of nitrogen indicates the presence of the sulfamate ion. Since excess barium ion may be assumed as being present from thepreviously mentioned removal of insoluble barium salts, a white precipitate of barium sulfate should form if the sulfamate ion is present. An alternate procedure is to add 4 M acetic acid dropwise to three drons of the orieinal unnknown solution prior t o the addi'tion of the 'odium nitrite solution. Any effervescence is allowed to continue to completion. The interfering carbonate, sulfite, and sulfide ions are eliminated. The sodium nitrite test solution is then added. Effervescence at this point is attributed to the reaction between sulfamic acid and nitrite ion.
